69ec718b62db5766e43552cadfc7b659c7025ba4 changed nova to start using
the network mtu value from neutron (if specified and > 0) when
plugging virtual interfaces. This allows us to finally remove the
deprecated 'network_device_mtu' config option from nova (probably in
Newton).
Add a release note to point out that the segment_mtu value in neutron
is now used to set the mtu value during vif plugging in nova.
Change-Id: Iebffb487cc837427b0ede35aea93ec5f9594165f
Related-Bug: #1549408